As a member of the herpesvirus family, cytomegalovirus (CMV) establishes a life-long latent infection in the human host. The virus can be reactivated to produce serious disease in immunosuppressed patients and can be transmitted from donor to recipient by blood transfusion and organ transplantation. Although the latent infection is thus central to the pathogenesis of CMV disease, progress in identifying the cells which are latently infected and in defining the replicative state of the latent virus has been slow. Relevant and extremely important information concerning this issue has been derived in a well-defined murine CMV (MCMV) model which closely mimics the features of latent human CMV infection. MCMV establishes a non-replicating latent infection in the spleen, kidney, heart and, as demonstrated in this proposal, in the liver and lung. The latent virus in the spleen can be induced to replicate by establishment of tissue explants or by co-cultivation of spleen cells with permissive mouse embryo fibroblasts in vitro. We and others have demonstrated that the latent MCMV infection in the spleen is maintained in cells within the splenic stroma or red pulp, possibly exclusively, although the specific cells involved have yet to be identified. Thus, we propose to prove or challenge a hypothesis based on currently available data in humans and in the mouse model that CMV latency is maintained primarily in the stromal compartment of different tissues, accounting from the frequent transmission of infection by transplantation of diverse organs. The low rates of transmission by blood transfusion would reflect periodic reactivation of the virus in the primary tissue repositories followed by permissive or abortive infection of blood leukocytes. The hypothesis will be tested using modern molecular methods (enzymatic amplification of viral nucleic acids extracted from tissues, amplification of viral DNA within cells and tissues by combined polymerase chain reaction and in situ hybridization technology developed at any institution, and by study of highly purified cell populations of relevant organs). The initial studies will be conducted in blood leukocytes, the spleen and the liver. Subsequent studies will involve the heart, lung, and kidney. The cells responsible for maintenance of latent MCMV infection will be identified and the extent of viral gene expression during the latent viral state, if any, will be characterized in cells and organs. These studies will delineate fundamental aspects of latent CMV infection which cannot be fully characterized in humans and which have great importance in the field of organ transplantation.